Many packaged items, particularly foods, have a finite shelf life. Since one cannot easily inspect the contents of an item through its packaging, it is customary to estimate when the item might reach the end of its shelf life, and to then print a corresponding expiration date on the item.
Mass-produced items are typically produced at different times. Since such items would normally have similar shelf lives, it follows that the expiration date to be printed on each item will vary with the production date of that item. In many cases, one obtains the expiration date by adding a fixed offset to the production date. However, more complex rules are sometimes used.
In general, the expiration date can be regarded as a function of the production date. Conventional marking devices typically determine the production date of an item, and then calculate the corresponding expiration date. This procedure is computationally intensive. Moreover, a marking machine is often used for different types of items, all of which have different rules for determining an expiration date.